The present invention relates to an ultrapure water producing system, and in particular, to an ultrapure water producing method and ultrapure water producing system capable of producing ultrapure water having a reduced amount of TOC (Total Organic Carbon) and a low electrical conductivity.
The water quality required for ultrapure water in a semiconductor manufacturing factory has become more and more strict in accordance with the microstructural progress of semiconductor devices. Particularly in a factory for manufacturing semiconductor devices of a degree of integration higher than that of 64 Mbit-DRAM (Dynamic Random-Access Memory), the TOC is reduced to a density of not higher than 1 ppb (Parts Per Billion) in the ultrapure water.
In general, an ultrapure water producing system is constructed of a pretreatment unit, a primary pure water producing unit and a secondary pure water producing unit. Then, the pretreatment unit adopts physicochemical methods such as coagulative sedimentation, coagulative filtration and coagulative pressurized floatation. Concrete examples of such pretreatment units are shown in FIGS. 16A and 16B. The pretreatment units shown in FIG. 16A and FIG. 16B adopt the methods of sedimentation and filtration that are both physicochemical treatments and are pretreatment units intended mainly for the removal of sludge in the water to be treated. Therefore, the pretreatment units shown in FIGS. 16A and 16B cannot be regarded as a treatment system for effectively treating organic nitrogen compounds that cause an increase in TOC density.
As the above primary pure water producing unit, there is a system of a combination of a reverse osmosis unit, an ion exchange resin unit and an infrared sterilization unit. As the above secondary pure water producing unit, there is one constructed of an ultraviolet oxidation unit, an ion exchange resin unit, an ultrafiltration membrane unit and so on.
According to the recent research, it has been discovered that the remaining TOC in the ultrapure water is caused by the organic nitrogen compounds coming from raw water. In this case, "raw water" means industrial water, underground water and the like. In contrast to this, drinking water and tap water are called "city water".
In general, for the method of treating the carbon compounds and organic nitrogen compounds and the like that cause an increase in TOC density in the water to be treated, a reverse osmosis unit, a special ion exchange resin unit or an ultraviolet oxidation unit, as described above, are used. However, according to the recent demand for a water quality of a TOC density of not higher than 1 ppb in ultrapure water, a pretreatment unit has also been developed that adopts a biotic treatment method utilizing an aerobic microbe in addition to the aforementioned generic organic matter treatment system (prior art reference of Japanese Patent Laid-Open Publication No. HEI 6-63592).
In accordance with the growing consciousness of the water resources, many ultrapure water producing systems have been proposed that collect waste water and reuse the same instead of limitlessly using the city water, industrial water, underground water and the like as raw water. A method has been proposed for producing ultrapure water by mixing raw water with waste water containing several parts per million of IPA (isopropyl alcohol) and acetone that serve as organic solvents and biologically treating the organic nitrogen compounds (prior art reference of Japanese Patent Laid-Open Publication No. HEI 6-233997). According to this ultrapure water producing method, the organic nitrogen compounds in the water to be treated are treated through a biotic treatment by the pretreatment unit and thereafter treated successively by a primary pure water producing unit and a secondary pure water producing unit, finally obtaining ultrapure water having a TOC density of not higher than 1 ppb. It is to be noted that an aerobic microbe is utilized and activated carbon is used as filler for the biotic treatment in the pretreatment unit, thereby treating the organic nitrogen compounds represented by urea.
An ultrapure water producing system that performs pretreatment by means of a microbe, is shown in FIG. 17. In this ultrapure water producing system, industrial water and collected water are introduced as raw water into a water receiving tank 1. Then, after a lapse of a retention time of at least three hours, the water is introduced into an upward flow type biotic decomposition unit 3 by a conveying pump 2. This upward flow type biotic decomposition unit 3 is stowed with activated carbon or the like, where an aerobic microbe is propagating. The water to be treated from the upward flow type biotic decomposition unit 3 is introduced into an aeration tank 4. Then, the water is aerated by aeration air discharged from an air diffusion pipe 5. A part of the water to be treated inside the aeration tank 4 circulates while being conveyed back to the upward flow type biotic decomposition unit 3 by an aeration tank pump 6. Another part of the water to be treated inside the aeration tank 4 is introduced into a pump pit 7 and conveyed successively to a filter unit 9 and a primary pure water producing unit 10 by a water conveyance pump 8.
Ultrapure water producing systems utilizing a biotic treatment with an anaerobic microbe and an aerobic microbe for the pretreatment unit, are shown in FIG. 18 and FIG. 19. In these ultrapure water producing systems, aerobic organic matter treatment sections 12 and 22 are arranged in upper portions of anaerobic organic matter treatment sections 11 and 21. Charcoal pieces 13 and 23 are stowed as filler, where an anaerobic microbe and an aerobic microbe are propagating. It is to be noted that the reference numerals 14 and 24 denote plastic fillers, the reference numerals 15 and 25 denote vinylidene chloride and the reference numerals 16 and 28 denote membrane filter tanks. The reference numeral 26 denotes a hydrogen peroxide oxidation tank, while the reference numeral 27 denotes a hydrogen peroxide decomposition tank.
As described above, the ultrapure water producing system is constructed of the pretreatment unit, the primary pure water producing unit and the secondary pure water producing unit. In general, the above primary pure water producing system is constructed by combining a reverse osmosis (RO) unit, an ion exchange resin unit, an ultraviolet sterilization unit and so on. The above secondary pure water producing unit is constructed of an ultraviolet oxidation unit, an ion exchange resin unit, an ultrafiltration membrane unit and so on.
The ion exchange resin unit of the above primary pure water producing unit exchanges ions in the water to be treated by the ion exchange resin. Then, after a lapse of a specified time, the ion exchange function is restored through regeneration by hydrochloric acid and sodium hydroxide. In the regeneration stage, mineral acid (hydrochloric acid or sulfuric acid) or sodium hydroxide are needed as a chemical, and regenerated waste water results having ion concentration. In regard to this regenerated waste water, acid waste water is generated when the ion exchange resin is regenerated by the mineral acid or generated alkaline waste water is generated when the ion exchange resin is regenerated by sodium hydroxide. Either type of waste water necessitates waste water treatment of neutralization or the like as well as waste water equipment for that purpose. A tank for reserving the mineral acid or sodium hydroxide, a pump and piping for conveying the chemical are also needed.
Accordingly, from the viewpoint of reducing the space and cost, an electric deionizing system, which obviates the need for the chemicals such as ion exchange resin regenerating chemicals and the installation of regenerating equipment having no direct relation to ultrapure water production, has started to attract attention.
However, the above prior art ultrapure water producing method and ultrapure water producing system has problems as follows.
First, in the ultrapure water producing system shown in FIG. 17, the aerobic microbe is used for the pretreatment, and therefore, reductions in the amount of carbon compounds and organic nitrogen compounds can be expected. However, since no anaerobic microbe is used, the system does not have the function of reducing the nitrate nitrogen attributed to the organic nitrogen compounds into nitrogen gas. This also leads to the problem that the electrical conductivity of the water to be treated can not be reduced at all. Furthermore, since the upward flow type biotic decomposition unit 3 is the upward flow type, the activated carbon serving as a filler moves, consequently causing a problem that the microbe propagating on the activated carbon comes off. Therefore, a filter membrane unit such as an ultrafiltration membrane unit for interrupting the microbe that have flowed out is necessary, which increases the initial cost.
According to the above ultrapure water producing system disclosed in the prior art reference of Japanese Patent Laid-Open Publication No. HEI 6-63592 and the ultrapure water producing method disclosed in the prior art reference of Japanese Patent Laid-Open Publication No. HEI 6-233997, which performs a biotic treatment by means of activated carbon used as a filler, the TOC density in the water to be treated reaches about 100 ppm when the collected water is made to join the raw water, and the aerobic microbe rapidly propagates, sometimes rapidly causing partial clogging of the activated carbon due to the abnormal propagation of the aerobic microbe. Particularly with regard to the abnormal propagation of the microbe attributed to IPA and acetone, there is the problem that effect of suppressing the above clogging even when the activated carbon is washed by frequent back washing. Even under the condition that the TOC density is not specifically high, the microbe propagating on the activated carbon disadvantageously comes off due to the flow of the activated carbon since the unit stowed with activated carbon has the upward flow. In view of the above, an ultrafiltration membrane unit or a micro filter unit for interrupting the microbe that have flowed out is provided, however, there is the problem that this ultrafiltration membrane unit or the micro filter unit is disadvantageously clogged.
Furthermore, the retention time of the water to be treated in the upward flow type microbe decomposition unit used for the aforementioned pretreatment unit becomes extremely long when the TOC density of the water to be treated is increased by orders of magnitude, and this leads to a difficulty in handling in terms of system size, i.e., the problem that the water quality cannot be secured.
A ultrapure water producing system has been proposed that is provided with a pretreatment unit having a biotic treatment means and a chemical oxidation means (prior art reference of Japanese Patent Laid-Open Publication No. HEI 7-284799). In the case of this ultrapure water producing system, the TOC can be further reduced since the chemical oxidation means is used in the pretreatment unit. However, since the biotic treatment means is concurrently used, when low-concentration organic waste water having a low TOC density of about 100 ppm is introduced as water to be treated, there is the problem that the microbe that has abnormally propagated disadvantageously clog the ultrafiltration membrane unit and the micro filter unit provided in the subsequent stages.
Furthermore, the ultrapure water producing system using the biotic treatment with the anaerobic microbe and the aerobic microbe for the pretreatment unit shown in FIG. 18 and FIG. 19 employs the charcoal pieces 13 and 23 that are greater in size than the activated carbon as a filler. Accordingly, there is a small possibility of the occurrence of clogging due to the microbe and the separation of the microbe due to the upward flow when low-concentration organic waste water having a low TOC density of about 100 ppm is introduced as water to be treated, so that a more stabilized biofilm is formed. However, the arrangement of the aerobic organic matter treatment sections 12 and 22 above the anaerobic organic matter treatment sections 11 and 21 causes a problem that the height of the treatment tank becomes considerably high. The use of the charcoal pieces 13 and 23 having an adsorbing ability inferior to the activated carbon for the carbon compounds and the organic nitrogen compounds in the water to be treated also causes a problem that the pretreatment unit of the ultrapure water producing system has a low TOC removal rate.
Furthermore, by implementing the retention carrier of the microbes by the charcoal pieces 13 and 23, the flow of the retention carrier and the accompanying separation of microbes can be reduced to a certain extent. However, since the water to be treated has the upward flow and downward flow, it is impossible to prevent the separation of the microbe by completely suppressing the flow of the charcoal pieces 13 and 23.
Furthermore, as described above, the electric deionization unit has merit in producing ultrapure water for the advantage that the unit obviates the need for chemicals such as ion exchange regenerating chemicals and the installation of a regeneration unit having no direct relation to the production of ultrapure water, possibly gaining wide market acceptance in the future. However, in order to use the electric deionization unit as the primary pure water producing unit, it is required necessary to securely treat in the pretreatment unit the organic matters, dissolved gas and so on, which are other than ions and reduce the deionizing function of the electric deionization unit. In this case, the above ions mean not only dissociated ions of Na, Ca, Mg, Cl, SO.sub.4 and the like but also the week ion components of CO.sub.2, SiO.sub.2 and the like.
In general, the electric deionization unit is installed subsequent to the reverse osmosis unit and is intended for treating the city water or the like that has a TOC density as a measure for the organic matter content of not higher than 2 ppm and a relatively high water quality. Therefore, when treating raw water having a lower water quality than that of city water, the ultrapure water producing system including the electric deionization unit also necessitates the reverse osmosis unit in the preceding stage of the electric deionization unit as well as an ultrafiltration membrane unit in the preceding stage of the reverse osmosis unit. Accordingly, this system disadvantageously has a high initial cost and running cost.